Function accepted 

pub fn accepted(response: &Response) -> bool

Whether the registrar actually performed an outbound registration (§6).

§6 requires a registrar that did to “include the ‘outbound’ option tag in a Require header field” of the 2xx. Checking it is what stops a UA from running keep-alives on a flow nothing is routing down, and from believing a NATed binding is durable when the registrar recorded an ordinary one.